Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser diodes at the transmitter. These laser diodes are tuned to operate at different wavelengths. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman pumping scheme. At the receiving end, the channels are usually separated from each other using thin film filter systems, to thereby enable detection by separate photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds or thousands of spectrally separated channels per fiber. Such configurations yield concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data for WDM systems is typically less than comparative non-multiplexed systems. This is because any amplification system required along the link can essentially be shared by all of the separate channels transmitted in a single fiber link. With non-multiplexed systems, each channel/fiber would require its own amplification system.
The economics pulling for WDM in the context of long-haul optical links is only one factor suggesting the long-term applicability of the technology. Another application concerns the dynamic routing of individual wavelength slots or channels in optical WDM networks with multiple network access nodes. Such network functionality requires devices that can add and drop specific channels in an optical link.
One problem that arises in the context of controlling tunable filters, and more specifically, Fabry-Perot tunable filters concerns the control algorithms for these MOEMS devices. Specifically, to increase the ease at which the results of a scan of the signal band of a WDM signal can be analyzed, it would be desirable if the change in the pass band of the tunable filter as a function of time were linear or near linear in frequency or wavelength.
In general, according to one aspect, the present invention concerns an optical filter system. This filter system comprises a tunable optical filter that scans a pass band across a signal band to generate a filtered signal. A filter tuning voltage generator generates a tuning voltage to the optical tunable filter. A photodetector generates an electrical signal in response to the filtered signal. Finally, a controller, that is responsive to the photodetector, triggers the filter tuning voltage generator.
According to a preferred embodiment, the voltage generator generates a tuning voltage to the optical tunable filter that improves a linearization of the tuning pass band as a function of time over, at least, a portion of the scan of the signal band. In one implementation, the controller maps a pass band wavelength as a function of voltage for the tunable filter. This can be done with a look up table. In another implementation, the controller maps a pass band frequency as a function of voltage for the tunable filter. This can be done with a look up table.
In general, according to another aspect, the invention also concerns an optical filter tuning process. This process comprises a tunable filter scanning a pass band across in a signal band to generate a filtered signal. A non-linear tuning voltage is generated as a function of time and this tuning voltage is applied to the tunable filter. Finally, the filtered signal is detected from the tunable filter.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.